The present invention relates to a method for fabricating a semiconductor device, and more particularly, to a method for fabricating a semiconductor device having a bulb-shaped recess gate.
Recently, as the integration scale of semiconductor device has been increased, a bulb-shaped recess gate structure that can increase a channel length of a cell transistor has received a great attention from many researchers. Hereinafter, with reference to FIGS. 1A to 1D, a method for fabricating a semiconductor device having such a bulb-shaped recess gate will be briefly explained.
FIGS. 1A to 1D illustrate a typical method for fabricating a semiconductor device. As shown in FIG. 1A, a device isolation structure 12 is formed in a substrate 11 to define an active region.
A plurality of bulb-shaped recesses 13 are formed in the active region of the substrate 11. In more detail, certain portions of the active region of the substrate 11 are etched to form a neck pattern 13A having a vertical profile. A spacer insulation layer (not shown) is formed over sidewalls of the neck pattern 13A. Then, the substrate 11 below the neck pattern 13A is subjected to an isotropic etching process using the spacer insulation layer as a barrier to form a bulb pattern 13B having a sphere profile. The bulb-shaped recesses 13 are formed with the neck pattern 13A and the bulb pattern 13B via the etching process performed by two steps.
As shown in FIG. 1B, a gate insulation layer 14 is formed over an entire surface of the device isolation structure 12 and the bulb-shaped recesses 13.
As shown in FIG. 1C, a polysilicon layer 15 for gate electrodes is formed over the gate insulation layer 14. The polysilicon layer 15 includes a conductive layer. Since a width of the neck pattern 13A formed at an upper portion of the corresponding recesses 13 is narrower than that of the bulb pattern 13B formed at a lower portion of the corresponding recesses 13, the polysilicon layer 15 first fills the neck pattern 13A before the polysilicon layer 15 completely fills the bulb pattern 13B. As a result, a polysilicon seam identified with a reference letter ‘A’ is generated inside the bulb-pattern 13B. This polysilicon seam ‘A’ shifts to the inside of the bulb pattern 13B through a subsequent thermal process. Particularly, the polysilicon seam ‘A’ shifts to the direction which a surface energy is decreasing so that it exists at a boundary portion between the bulb pattern 13B and the gate insulation layer 14. The polysilicon seam ‘A’ existing at the boundary portion between the bulb pattern 13B and the gate insulation layer 14 induces a potential drop phenomenon or generates a mis-operation of a transistor due to a deformation of a seam distribution.
As shown in FIG. 1D, a gate hard mask layer is formed over the polysilicon layer 15 having the seam ‘A’ and then, the gate hard mask layer and the polysilicon layer 15 are selectively etched. Accordingly, a gate hard mask pattern 16 and a patterned polysilicon layer 15A forms a gate pattern. A metal layer or a metal silicide layer (not shown) may be disposed between the patterned polysilicon layer 15A and the gate hard mask pattern 16.
FIGS. 2A and 2B illustrate a generation and a shift of a polysilicon seam at a typical semiconductor device. As shown in FIG. 2A, a polysilicon layer is not uniformly formed inside a bulb-shaped recess but a seam that is not filled with the polysilicon layer is generated inside the bulb pattern of a bulb-shaped recess. The reason for the generation of the seam is that a width of a neck pattern of the bulb-shaped recess is narrower than that of the bulb pattern of the bulb-shaped recess.
As shown in FIG. 2B, the seam shifts to the inside of the bulb pattern through a subsequent thermal process and particularly, shifts to an inner wall of the bulb pattern. Various distributions of the shift of the seam are also possible.
For the typical bulb-shaped recess gate fabrication process, in the case of forming a polysilicon layer over a bulb-shaped recess to form a gate electrode, a seam is generated inside a bulb pattern due to the structure property of the bulb-shaped recess. The seam shifts via a subsequent thermal process and thus, various seam distributions may be shown. Accordingly, the seam affects a device property such as mis-operation of a transistor.